1. Field of the Invention
The present invention relates to a pushbutton telephone system capable of holding a telephone connection that has been established by a call.
2. Description of the Prior Art
There has been known a pushbutton telephone system having a main controller and a plurality of pushbutton telephone sets. The main controller controls connections between the pushbutton telephone sets and also connections between the pushbutton telephone sets and a plurality of telephone lines.
FIG. 1 of the accompanying drawings shows each of such pushbutton telephone sets. As shown in FIG. 1, the pushbutton telephone set comprises a handset 51, a liquid crystal display (LCD) panel 52 for displaying operation data, a pushbutton dialing pad 53, a hold key 55, a plurality of line keys 56, 57, 58, 59, and an extension key 60.
When the pushbutton telephone set shown in FIG. 1 responds to an incoming call routed through any one of the telephone lines, the user of the pushbutton telephone set picks up the handset 51 and presses one of the line keys 56.about.59 which corresponds to the telephone line that carries the incoming call, thereby establishing a telephone connection to the calling party through the telephone line and a telephone circuit network connected thereto. Occasionally, during the call, the user wishes to make an outgoing call through another telephone line or an extension line in order to make an inquiry, for example, as required by the incoming call. At this time, the user presses the hold key 55 to maintain the telephone connection initiated by the incoming call. Then, the user makes an outgoing call by pressing a desired one of the line keys 56.about.59 and the extension key 60. In this process of setting up a new connection, the user may first press one of the line keys 56.about.59 and the extension key 60 without pressing the hold key 55 due to an oversight. If the user presses one of the line keys 56.about.59 and the extension key 60 without holding the original call, then the telephone line carrying the original call is disconnected rather than being maintained. Such a drawback would be eliminated if all the telephone lines that carry calls were maintained when one of the line keys 56.about.59 and the extension key 60 is pressed during the calls. However, this solution would be disadvantageous because any other telephone lines that are not in service would also be maintained and not available for calls.
Because of the above shortcoming, care should be exerted in using the conventional pushbutton telephone system particularly when a new connection is to be set up while a previously established call is being maintained.